Agriculture Reference
In-Depth Information
The disdrometer described by Hall and Calder (1993) is also based on optical
principles. An annular light beam is received by a photodiode detector that generates
current pulses whose heights are linearly related to the diameter of drops passing
through the sampling volume. There are special algorithms to account for drops that
are partially outside the sampling volume or/and partially masking each other.
Another disdrometer is based on a transducer exposed to rain (Disdromet, Basel,
Switzerland); during rain, the vertical transfer of momentum for each impacting
drop produces a pulse whose height is related to the drop diameter by a power law.
(b) Integral parameters and duration characteristics
The kinetic energy (or power) and intensity of rainfall can be calculated using the DSD
measurements. The drop velocity is either measured (with a spectropluviometer) or
estimated using empirical relationships (Ulbrich, 1983; Madden, 1992). Rain amount
and intensity are usually measured directly (Viton, 1990). Based on the two DSD
parameters linked to the type of rainfall (convective, stratiform, etc.), a generalized
power law relationship between rainfall power (i.e. kinetic energy per unit area and per
unit time) and intensity was derived (Salles
et al.
, 2002; see Equation 28 and Table
2). During heavy showers or storms, errors may occur in measuring rainfall intensity
that are relevant to rain-splash. Water dumping from one of the two tipping bucket
rain-gauge chambers may not be properly recorded since the tipping movement of
the vessel is not instantaneous. Tipping-bucket gauges produce electrical pulses
which are easily computerized to calculate total amounts of rain, temporal changes
in intensity, maximum and mean intensities over different time periods and periods
of constant rainfall intensity. When the time interval between rainfall amount
measurements is relatively long (rarely <1 hour), the histogram of rainfall intensity
can be useful.
A type of rain gauge operates by detecting (rather than by measuring threshold
amounts of water) and delivering an analogue voltage output to estimate instantaneous
rain intensity. This could be very useful for measuring the duration of periods with
rain intensity above given threshold values within a rainfall episode. For studies where
limited equipment is available, a direct measurement of rain duration in association
with rain amount data could be of use. The rain duration sensors consist of a grid
(similar in principle to leaf wetness sensors) composed of two electrodes separated
by a space that is bridged by rain water. The electrodes are heated continuously so
that fog deposition and dew formation are not recorded (Viton, 1990).
16.6.2 Methodology for measuring potential rain-splash
(a) Measurements using a standard target
A simple instrument for the direct measurement of maximum splash height was
developed by Shaw (1987). This 'splash meter' consists of an annular reservoir
placed on the ground filled with a UV-fluorescent dye with a vertical cylinder
